A variety of silicone-modified novolak resins are known in the art. Many documents including JP-A S58-122922, for example, describe novolak resins in which phenolic hydroxyl groups are silicone modified. With these structures, the characteristics of silicone compounds are outstandingly exerted because the characteristic of phenolic hydroxyl group which is one of characteristics of novolak resins is excluded. Another class of silicone-modified novolak resins includes siloxane-modified novolak resins having a conjugated double bond as described in JP-A H03-97710. Since the siloxane modifier used in this document is an organopolysiloxane having hydrosilyl groups at both ends, intramolecular and/or intermolecular bonds are formed, posing limits to molecular motion. In addition, since the resin has a conjugated double bond and an epoxy group within a molecule, the low stress characteristic of silicone is compromised despite good mechanical properties after final curing. Also, novolak type phenolic resins which are silicone modified by block copolymerization are known from JP-A H05-59175, for example. Since they are obtained from copolymerization of telechelic novolak resin with telechelic polysiloxane, they have the drawbacks that preparation of such intermediates is cumbersome, and the alkali dissolution rate is slow because of the structure containing polysiloxane in the backbone. It would be desirable to have a novolak resin having a high alkali dissolution rate and low stress.
Since resist films of photoresist materials using prior art novolak resins will crack during or after electrolytic plating, it is desired to prevent such cracking. JP-A H10-207057 and JP-A 2004-198915 propose to add a specific additive to the photoresist material to mitigate the stress in the film for suppressing cracking. However, since the additive used in this method is a water-soluble or alkali-soluble compound, the resist film undergoes a thickness loss during long-term development. In the subsequent electrolytic plating step, this gives rise to the problems that the desired plating thickness is not reached, and the additive will be dissolved into the plating solution. These problems are inherent to the attempt to avoid structural defects of the resin itself by adding the additive. It would be desirable to have a resist material which essentially overcomes the above-mentioned drawbacks.
With respect to the silicone-modified novolak resins, U.S. Pat. No. 6,429,238 describes the modification of a novolak copolymer with a silicone. Novolak resin segments are added to both ends of the silicone. When such a silicone-modified novolak resin is used to formulate a positive resist composition, the resist composition is substantially undevelopable because the dissolution rate in alkaline aqueous solution, especially 2.38 wt % TMAH aqueous solution is significantly retarded.